The use of hand-held computers continues to expand as their capabilities increase and the cost associated with ownership and use of such computers declines. Popular commercially available hand-held or palmtop computers include the Casio B.O.S.S..TM., Sharp Wizard.TM., and the Hewlett-Packard and Psion series of computers. Hand-held computers have traditionally been used for data management such as day planning, but their use is expanding to electronic mail, spreadsheet, and other tasks.
The most significant advantage of hand-held computers is their relative light weight and small size. In an effort to minimize the size of hand-held computers, they typically do not include a full-size keyboard. Ironically, this is perhaps also their most significant disadvantage, because data entry on the smaller keyboards is not as efficient as it is on a desktop computer. A solution to this problem is to interconnect the hand-held computer with a desktop computer using a cable, input the data to be entered into the desktop computer, and then transfer the input data to the hand-held computer. Coupling the two computers also allows data on the hand-held computer to be transferred to a desktop computer for archival purposes.
Many of the available hand-held computers operate at voltage levels that are incompatible with the serial ports on desktop computers. For example, logical zero and one on a hand-held computer may be at zero and five volts, respectively, whereas on a desktop computer the voltage levels may be in the range of negative 15 volts for a logical zero and positive 15 volts for logical one. Thus, the voltage levels must be convened for communications to be successful. A number of computer data interface solutions have been designed to provide compatibility between hand-held and desktop computers. An early solution was a cable that connected the Sharp Wizard to an IBM.RTM. or IBM-compatible personal computer. However, the cable was inconvenient because the voltage conversion circuitry was contained in a relatively bulky housing and was externally powered. A more advantageous solution is disclosed in U.S. Pa. No. 5,157,769, assigned to Traveling Software, Inc., the assignee of the present application.
U.S. Pat. No. 5,157,769 (the '769 patent) discloses a computer data interface for connecting a hand-held computer and a desktop computer. The computer data interface includes a cable having connectors at each end. A voltage level conversion circuit is mounted adjacent to one of the connectors for receiving data signals from the hand-held computer and transmitting the signals to the desktop computer at voltage levels compatible with the desktop computer. Similarly, the level conversion circuit receives signals from the desktop computer and transmits the signals to the hand-held computer at voltage levels compatible with the hand-held computer. The level conversion circuit is powered by the desktop computer to prevent draining the batteries of the hand-held computer.
FIG. 2 of the '769 patent illustrates a voltage level conversion circuit for transmitting data between an IBM or IBM-compatible personal computer (PC) and a Sharp Wizard hand-held computer. The circuit utilizes a Motorola.RTM. MC 145406 integrated circuit to perform voltage conversions necessary to allow the two computers to communicate. FIG. 3 of the '769 patent illustrates a level conversion circuit that may be used to transmit data between a PC and a Casio B.O.S.S. (Boss) hand-held computer. The primary components in the level conversion circuit of FIG. 3 include a number of resistors 42, a transistor 44, a diode 48, and an isolation circuit 52. The transistor 44 has its base connected to the transmit data TxD line (pin 2 of the PC), its emitter connected to ground, and its collector connected to the positive voltage source V+ of the PC through one of the resistors 42 as well as the receive line R of the Boss through a resistor 50. The isolation circuit is coupled between the collector through two resistors 42 and ground on the Boss.
Briefly, the operation of the level conversion circuit of FIG. 3 is as follows. When the TxD line (pin 2) is low, positive voltage is applied to the receive line R of the Boss, i.e., from V+ with the voltage being clamped by the diode 48. When TxD goes high, the transistor 44 turns on, sinking current to ground and pulling R low. The isolation circuit is needed to disconnect the ground of the Boss from the PC to ensure that the PC does not drain the batteries of the Boss. This may result because pin 20 will be low when the PC is turned off, and thus current may be sourced from R through the resistors 50 and 42 to pin 20.
Transmission in the opposite direction is accomplished using a capacitor 34, a diode 36 and an operational amplifier (op amp) 40. The positive terminal of the op amp 40 is connected to V+ through a resistor 42. The negative terminal is connected directly to the transmit line of the Boss. The output of the op amp is connected to the receive line RxD (pin 3) of the PC. The capacitor 34 is charged with a negative voltage using the TxD line (pin 2). When TxD is low, the output of the op amp is at V+. When TxD is high, the output of the op amp is switched to the negative voltage supply stored in capacitor 34.
A more recent computer data interface to become commercially available is the model CE-137T level converter manufactured and sold by Sharp Corporation. The CE-137 T level converter may be used to transfer data between the Sharp Wizard and either a Macintosh computer or a PC in a single interface. Prior to the CE-137 T level converter, available computer data interfaces typically were designed to connect a hand-held computer to only one of the two types of computers, either Macintosh or PC's, i.e., a first interface would be required for data transfer if a user had a Macintosh computer, and a second, different interface would be required for data transfer if a user had a PC. The CE-137 T level converter includes two Motorola integrated circuits to provide voltage conversions between a hand-held computer and both Macintosh computers and PC's.
The computer data interfaces discussed above successfully accomplish voltage level conversions to allow a hand-held computer to communicate with a desktop computer. However, prior art computer data interfaces have typically required integrated circuits. In earlier models, the integrated circuits were powered externally. In the '769 patent and CE-137 T level converter, the integrated circuits were powered by the desktop computer. The invention is a simplified circuit design that does not require use of integrated circuits or external power, presenting cost, space, and power savings. The power savings may be significant if the desktop computer is also battery powered.